1. Field of the Invention
The invention relates to a support device for submerged electrodes in the bath of an electroplating installation, and also to the current supply to these electrodes, particularly in the case of electrodes that are expendable, soluble anodes and that have to be exchanged during electroplating. An example of an installation of this type is an installation for zincplating steel strips in a chloride-based electrolytic bath.
2. Discussion of the Background
Cells are usually used as a zincplating installation for strips. These cells are typically referred to as "radial" cells. A shown in FIGS. 1-6, the cells may include mechanisms for passing the strip 100 to be coated through a bath 5. Such a mechanism may include, for example, a strip carrying roller 1 at least partially submerged under the surface level of the bath 5; support devices 2, 4 serving both as support for, and as current supply to, the submerged electrodes 3; and mechanisms for causing an electric current to flow between the strip 100 to be coated (serving as cathode) and the electrodes 3 (serving as anodes) on said support devices 2, 4. For zincplating strips in chloride medium, soluble anodes made of zinc or a zinc alloy are generally used.
The electrodes 3 (or soluble anodes) are formed of curved bars turned toward the roller 1 along the direction of travel of the strip 100. The electrodes 3 are also grouped in sets of electrodes 3 positioned side by side so as to form a cylindrical generator portion partially enveloping the roller 1 in the electroplating bath, as illustrated in FIGS. 1 and 2. The arrows shown in FIG. 4 illustrate how an electroplating electric current may flow.
As illustrated in FIGS. 1-3, each support device 2, 4 is common to all of the electrodes 3 of a corresponding set. In the example given in FIGS. 1-3, each support device 2, 4 is formed of a beam positioned transversally to the travel path of the strip 100 on which all the electrodes 3 of a given set rest. Each electrode 3 is held against the beams by an electrode hook 31.
The mechanical and electrical contact between an electrode 3 and its corresponding support device 2, 4 defines an interface 6 between a resting surface 6A of the electrode 3 and a corresponding bearing surface 6B of the support device (see FIGS. 5 and 6). Since the electrodes may be expendable (as in the case of soluble anodes), their thickness varies (see FIG. 3) according to the level of wear, and it may be necessary to change the electrodes 3 during electroplating as they dissolve. During electroplating, the electrodes 3 in the same set are slid along their corresponding support devices 2, 4 in the directions A and B respectively (see FIG. 3) in order to remove a worn electrode 3 at one end of the beam while creating a place for a new electrode 3 at the other end. To make the sliding of the electrodes 3 possible, each electrode 3 rests against its corresponding support device 2, 4 at the interface 6 only under the force of its own weight. Thus, the electrodes rest freely against their respective support device.
The support devices 2, 4 also serve to supply the electrodes 3 with electric current for electroplating. It has been noted that the electrical contact resistance at the interface 6 produces large energy losses. Given the weight of each electrode 3, the pressure exerted at the interface 6 on the support device generally does not exceed 10.sup.4 Pa or 1 Newton per cm.sup.2 of the bearing surface. During electroplating, the circulation of the bath 5 in the installation may cause this resting pressure to fall below 0.1 Newton per cm.sup.2 of the bearing surface (10.sup.3 Pa). As used herein, the term "bearing surface" means the total surface area at the interface 6 between the electrode 3 and the respective support device 2, 4.
Energy losses resulting from contact resistance at the interface 6 become particularly significant when the current density exceeds 0.025 A/mm.sup.2 at the interface 6, particularly when the resting pressure is less than 10.sup.4 Pa, and even more so when the resting pressure is less than 10.sup.3 Pa. The increasing losses in energy appear to stem from a slight lifting of the anodes under the effect of the electric current, requiring the electric supply current of the electrodes 3 to pass in transit through the bath 5 interposed at the interface and causing gas emissions, e.g., emissions of chloride, at this location. This phenomenon is schematized in FIG. 6.
The beams that serve as the support devices 2, 4 are generally formed of resin-impregnated graphite. This material wears and deteriorates as a result of two phenomena: first, wear caused by the friction of the electrodes sliding on the beam; and second, fissuring due to heating and/or gas emissions caused by the electrical contact losses described above.
A graphite-based material which resists wear well is generally less resistant to fissuring and vice versa. Therefore, it is difficult to find a good compromise when choosing graphite material, and it remains necessary, regardless of this choice, to replace the beams regularly which represents a significant economic handicap.